Fuel cells are energy converters that, unlike heat engines which go through heat energy and kinetic energy processes, include reacting fuels such as natural gas and hydrogen with oxygen in the air through a solid electrolyte and continuously and directly obtaining electric energy from chemical energy possessed by fuels. Among them, solid oxide fuel cells are fuel cells that operate as cells including a solid oxide (ceramic) as a solid electrolyte, a fuel electrode as a negative electrode, and an air electrode as a positive electrode. Further, solid oxide fuel cells are known as having an advantage that a high energy conversion efficiency can be obtained.
In solid oxide fuel cells, the output per unit cell is so low that power generation is carried out by enhancing output through connection of a plurality of unit cells in series. Members through which adjacent unit cells are electrically connected are called “interconnectors.” Interconnectors using ceramics as materials, hereinafter referred to also as “ceramic interconnectors”, are known. Gas sealing properties high enough to prevent gas permeation, electrical conductivity, oxide ion insulating properties, and adhesion to solid electrolyte are required as properties of ceramic interconnectors.
In general, a ceramic interconnector cannot provide satisfactory electrical conductivity unless the thickness is small, for example, approximately not more than 100 μm. When an attempt is made to form a ceramic interconnector having a reduced small thickness so as to obtain satisfactory electrical conductivity on a surface of porous electrodes such as fuel electrodes and air electrodes, there is a possibility that the ceramic interconnector is disadvantageously incorporated into the porous electrode. This leads to a disadvantage of a possibility that the ceramic interconnector cannot be formed or a possibility that, even when the ceramic interconnector can be formed, the thickness is so small that satisfactory gas sealing properties cannot be obtained.
When the gas sealing property of the ceramic interconnector is low, the fuel gas is disadvantageously leaked from the fuel electrode side of the ceramic interconnector to the air electrode side, resulting in mixing with air. In order to enhance the gas sealing property of the ceramic interconnector, the denseness of the ceramic interconnector should be increased. To this end, the ceramic interconnector should be densely sintered. When the electrical conductivity of a ceramic interconnector is low, the resistance of the ceramic interconnector is so high that the output of the fuel cell is disadvantageously lowered. Further, when the oxide ion insulating property of a ceramic interconnector is low, the oxide ions are disadvantageously leaked from the air electrode side to the fuel electrode side of the interconnector, leading to a lowered efficiency of the fuel cell. In addition, when the adhesion between the solid electrolyte and the ceramic interconnector is low, disadvantageously, gaps such as cracking occur between the solid electrolyte and the ceramic interconnector, resulting in leakage of the fuel gas through the gaps.
Lanthanum chromite (LaCrO3)-based interconnectors have widely been used as materials for the ceramic interconnector. It is known that the LaCrO3-based interconnectors have a high electrical conductivity but cannot be sintered without difficulties. Further, since chromium (Cr) is contained, there is a possibility that the so-called Cr poisoning occurs.
Further, SLT-based interconnectors represented by SrLaTiO3−δ have widely been used as materials for ceramic interconnectors. It is known that the SLT-based interconnectors have lower electrical conductivity but have better sinterability as compared with the LaCrO3-based interconnectors. In the SLT-based interconnectors, for example, the electrical conductivity is developed by replacing Sr site in the crystal lattice of SrTiO3, that is an insulator, with lanthanum (La) to give SrLaTiO3−δ (SLT), thereby converting a part of Ti4+ in Ti site in the crystal lattice of SrLaTiO3−δ (SLT) to Ti3+. δ is a value that is required to meet a neutral condition of the electric charge.
JP2008-270203A (PTL 1) aims to provide an SLT-based interconnector that simultaneously realize an improvement in electrical conductivity and an improvement in adhesion to an electrolyte layer while maintaining good airtightness. In order to realize this object, this patent literature describes that the ceramic interconnector has a two-layer structure of an airtightness-oriented portion formed on the fuel electrode side and an electrical conductivity-oriented portion that is formed on the air electrode side and has a higher electrical conductivity than the airtightness-oriented portion. Further, FIG. 2 in this literature shows that a solid electrolyte is formed between a fuel electrode in adjacent one power generation element and a fuel electrode in the adjacent other power generation element.
Nevertheless, when an air electrode 205 in adjacent one power generation element becomes close to a fuel electrode 302 in the adjacent other power generation element as in the conventional solid oxide fuel cell stack 280 illustrated in FIG. 8, oxide ions generated at the interface between the air electrode 205 and the solid electrolyte 204 in the one power generation element flow into the fuel electrode 302 in the other power generation element. This causes the formation of a counter cell that generates electromotive force in a direction opposite to electromotive force originally generated in the power generation element, leading to lowered power generation performance.
JP2015-064961A (PTL 2) describes a solid oxide fuel cell including: an insulative and ion-nonconductive first intermediate layer provided on a solid electrolyte in one power generation element and between an air electrode in one power generation element and a solid electrolyte in the one power generation element; and an insulative and ion-nonconductive second intermediate layer provided on a solid electrolyte in the other power generation element and between an air electrode in the one power generation element and the solid electrolyte in the other power generation element. Further, it is described that the first intermediate layer and the second intermediate layer may be in contact with an interconnector. PTL 2 describes that the solid oxide fuel cell can suppress the formation of a counter cell and can improve power generation performance.
In the solid oxide fuel cell described in PTL 2, however, since a part of the interconnector is covered with the solid electrolyte, a lowering in power generation performance cannot be effectively suppressed without difficulties.
Accordingly, a further improvement in the manufacture of a solid oxide fuel cell stack including power generation elements that can realize a high power generation output while suppressing the formation of a counter cell has been demanded.